Wind energy research: crucial for takeoff

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1 Downloaded from orbit.dtu.dk on: Dec 23, 2015 Wind energy research: crucial for takeoff Lundtang Petersen, Erik; Sørensen, Jens Nørkær Published in: Engineering challenges Publication date: 2009 Document Version Publisher final version (usually the publisher pdf) Link to publication Citation (APA): Lundtang Petersen, E., & Sørensen, J. N. (2009). Wind energy research: crucial for takeoff. In C. B. Hansen (Ed.), Engineering challenges: energy, climate change & health. (pp ). Kgs.Lyngby : Technical University of Denmark. (DTU research series). General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal? If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

2 Engineering challenges energy, climate change & health

3 Engineering challenges energy, climate change & health

4 Previous publications in the DTU research series: Bridging from technology to society (2004) Engineering challenges energy, climate change & health Editor Carsten Broder Hansen Text editing David Breuer Project supervisor Helge Elbrønd Jensen Project manager Tine Kjær Hassager Photographs Carsten Broder Hansen: pages 6/7, 12, 18/19, 22, 40/41, 52/53, 62/63, 72/73, 76, 77, 82/83, 86, 88, 94, 96, 98, 99, 116/117, 128/129, 134, 160/161, 170, 184, 200/201, 222/223, 230, 244/245 Carsten Dam-Hansen: page 56 Colourbox: pages 86, 99, 106/107, 115, 125, 180/181, 183 DTU: pages 135, 192, 198 EFDA-JET: page 102 Lauritzen Group: page 10 Lisbeth Holten: page 26 Thomas Fryd, courtesy of Grønttorvet København: page 182 Thomas Glasdam Jensen, Guisheng Zhuang: page 242 Google Earth: page 35 Klaus Holsting: page 66 Brian MacKenzie: page 153 Ninell P. Mortensen and James P. Nataro: pages 190/191 NASA: pages 97, 132 NASA/ESA/AURA/Caltech: page 165 Nicky Pløk/Uni-fly: pages 30/31, 37 Bjarne W. Olesen: page 79 Peder Skafte-Pedersen, Joanna Lopacinska, Martin Dufva: page 239 Søren Skou Nielsen, Pelle Ohlsson, Thomas Glasdam Jensen: pages 232/233 Detlef Snakenborg: pages 232/233, 235 Nikolaj Vynne: page 198 Photos of authors Carsten Broder Hansen, DTU, Klaus Holsting, Jesper Scheel Artwork Glynn Gorick: pages 148/149, 154, 155 NASA: pages 138/139 Graphics Møllers Tegnestue Layout Grethe Kofoed/Artcome Printing Svendborg Tryk A/S Printed in Denmark ISBN by the Technical University of Denmark All rights reserved For more information, contact: Technical University of Denmark Anker Engelunds Vej 1 DK-2800 Lyngby Denmark Tel Fax Web: 2 TECHNICAL UNIVERSITY OF DENMARK

5 Preface Action, Boldness & Creativity Action, boldness and creativity are the ABC of engineering. The refuge of academia, around which we all must stand guard, is a prerequisite for the free flow of thought, the critical solitary reflection and the inner secluded contemplation from which new ideas are born. Ideas of the individual that must be integrated with the ideas of others to form groundbreaking trains of thought, not only lifting the spirit of humanity but also bettering the human condition, indeed giving promise of a better future for humankind. The academic refuge can of course be abused in wasteful daydreaming, idle pastimes or even academic arrogance. All of us who detest such abuses must, in our defense of academic freedom, continuously fight to secure its ideals. However, even uncompromised, academic freedom cannot ensure the delivery of what academia can offer society, neither to the academic society nor more importantly to the society at large. Especially in science and technology, more, much more, is required. This much more is not to be solicited from society; we must solicit it from ourselves in the form of dedicated action. Passive reflection must be paired with active observation; interaction between theory and practice must be promoted; iteration between building models and experimental design must be employed; and dialogue between deduction and induction must be practiced. To be constructive, these endeavors require not only the abilities of the professionally trained researcher and the eager curiosity of the natural-born scholar but also a certain boldness. If limits are not tested, if well-known truths are not challenged, if mental faculties are not exercised vigorously, then the challenging questions posed even the most important ones are reduced to being solely academic. Scientific independence, a sibling to academic freedom, is an intellectual ideal very sensitive to its circumstances, and it may similarly wither away if it is left to self-indulgence and vain pride. Science and technology as research fields must responsibly and ethically live up to the highest standards of academic freedom and scientific independence to remain the justly admired disciplines they are. Nevertheless, the intercourse between science and technology bears a much larger promise. Adding genuine creativity to action and boldness brings us to the pinnacle of engineering. Perhaps the fascinating wonders of engineering can only be grasped if the ABC ingredients are recognized and understood in this context. Further, perhaps this insight is also the key to realizing why the wonders of engineering harbor the only possible answer to the pressing challenges posed by today s mounting problems and the only credible hope for a sustainable and livable future on this planet. I hope the readers of this book will be impressed, as I have been, by how DTU researchers actually unfold the ABC of engineering. We will join forces with colleagues from all over the world to meet the engineering challenges of tomorrow, especially those related to energy, climate change and health. Lars Pallesen President DTU

6 Contents Page Marine structures: consuming and producing energy... 6 Preben Terndrup Pedersen & Jørgen Juncher Jensen Sustainable energy catalysis is part of the solution Ib Chorkendorff Wind energy research: crucial for takeoff Erik Lundtang Petersen & Jens Nørkær Sørensen Fuel cells and electrolysis for converting renewable energy Mogens Mogensen & Søren Linderoth Photonics engineering in a new light Lars Dittmann & Paul Michael Petersen Power plants: energy and environmental issues Anker Degn Jensen & Peter Glarborg Buildings: consuming and conserving energy Bjarne W. Olesen & Carsten Rode Biorefineries: converting biomass into valuable products Irini Angelidaki & Kim Pilegaard Computer simulation: dynamical systems and multiscale modeling Per Christian Hansen, Jan S. Hesthaven & Jens Juul Rasmussen Waste and climate energy recovery and greenhouse gases Thomas H. Christensen Forecasting and optimizing transport Oli B.G. Madsen & Otto Anker Nielsen Towards a new grid infrastructure: Denmark as a full-scale laboratory Jacob Østergaard 4 TECHNICAL UNIVERSITY OF DENMARK

7 Page Changing energy systems to prevent climate change Kirsten Halsnæs & Hans Larsen Can we sustain fisheries as the climate changes? Brian R. MacKenzie & Keith Brander The adventurous journey of Spaceship Earth Henrik Svensmark Process design and production of chemicals, food ingredients, fuels and pharmaceuticals: multidisciplinary chemical engineering Anne S. Meyer, John M. Woodley & Rafiqul Gani Nutrition, genetics and health Inge Tetens & Ulla Vogel Microorganisms the good, the bad and the indispensable Lone Gram & Frank Aarestrup Small molecules and medicine Thomas E. Nielsen New-generation vaccines combining immunology with organic chemistry and bioinformatics Peter M. H. Heegaard & Ole Lund New methods of ultrasound imaging the tumbling blood Jørgen Arendt Jensen Microfluidic devices for health applications Jörg P. Kutter An engineering approach to health care Peter Jacobsen, Martin Grunow & Henning Boje Andersen DTU

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9 Marine structures: consuming and producing energy Preben Terndrup Pedersen & Jørgen Juncher Jensen

10 Seventy percent of the surface of the Earth is water. This space is immensely important for maintaining a reasonable standard of living for an ever-increasing population. Utilization of the sea is vital for the following reasons. The sea has huge importance for global exchange of goods; current global trade could not take place without using oceans for transport. The sea has great potential for the exploration and future cultivation of living resources in the form of fish and plankton. The sea and the seafloor have large reservoirs of raw materials and hydrocarbons. The sea receives 70% of Earth s primary sustainable energy source: radiation from the sun. This thermal energy can be harvested in the form of thermal, wind, current or wave energy, salt gradients and other forms. Exploiting this potential requires marine structures. Fig Cross-section of a very large crude oil carrier Shipping: energy and environment Ships are by far the largest and most complex mobile structures humans build. Their mission determines their size and main dimensions. In addition to the basic functional considerations influenced by cargo and routes, such requirements as low resistance, high propulsive efficiency, navigational limitations on draft and beam and stability influence the choice of dimensions and layout. Ship structures are therefore under constant development. Due to progress in research and continuing efforts towards more optimal solutions, ship transport has become the most environmentally friendly form of transport measured in tons per kilometer. An example is the very large crude oil carriers with cargo capacity between 220,000 and 320,000 tons (Fig. 1.1). For these vessels, the mass of the hull plus machinery and equipment is less than 15% of the mass of the oil carried (deadweight). No other means of transport has such a low ratio between self-weight and deadweight. Ships transport about 90% of global trade in raw materials and finished goods, accounting for about 4% of total CO 2 emissions. In this global picture, Danish shipping plays an important role. Ships registered in Denmark carry 10% of global trade measured in value. Even though marine transport is relatively very energy efficient, as demonstrated above, the ships registered in Denmark emit as much CO 2 as the rest of Denmark. The energy efficiency (grams of CO 2 per tonmile) strongly improves not only by reducing speed but also by increasing the size of the vessel (Fig. 1.2). Larger vessel sizes also strongly reduce the capital cost and the personnel cost per deadweight ton. This has been a main driver in the trend towards larger and larger ships. Container ships are an example. The first specially designed container ship appeared in 1960 and could carry foot equivalent units (TEU). In 1988, the size of container ships increased to about 4500 TEU, requiring transcending the existing maximum width of the Panama Canal (Panamax). Odense Lindø Shipyard was the first to design and build a series of post-panamax-size ships that could carry about 8000 TEU. These ships were the largest container ships until the delivery in 2006 of Emma Maersk, also from Odense Lindø Shipyard, as the first in a series of seven sister ships. Emma Maersk can carry more than 11,000 TEU; the deadweight is 157,000 tons, the main engine 80,080 kw and the maximum speed about 25.5 knots. These ships are the largest container vessels (length 398 m, width 56.4 m, depth 30.0 m) in the world fleet and have an unrivalled fuel efficiency index of 125 grams of CO 2 per TEU-mile. The structural design of such large vessels poses several technical challenges. The size has increased 8 TECHNICAL UNIVERSITY OF DENMARK

11 Container ships Service engine loading = 85% of maximum continuous rating Energy efficiency design index (grams of CO 2 per ton-mile) Panamax Postpanamax Panamax normal speed Panamax 10% speed reduction Panamax 20% speed reduction Post-panamax normal speed Post-panamax 10% speed reduction Post-panamax 20% speed reduction ,000 50,000 75, , , , ,000 Scantling deadweight (tons) Fig Energy efficiency design index in grams of CO 2 per ton-mile as a function of size for different speeds. Panamax: maximum width of the Panama Canal. so rapidly that there has been no feedback from service experience. Thus, these ultra-large steel structures have to be designed solely by direct calculation. Several unsolved challenges are associated with the design based on the basic mechanical principles of such large and fast ships. All the important structural loads on ships vary with time. The most important loads are caused by wave-induced motion of the water around the ship and the resulting motion of the ship itself. When ships increase in size and materials with greater strength are used, hull flexibility plays an important role in the response of the vessel. The natural frequencies of the hull girder become so small that considering the ship hull as a rigid body is not sufficient in calculating the wave-induced loads. The effects of structural hull deformation become important. That is, several hydroelastic effects must be considered for ultra-large container ships. Since container vessels have very open deck areas to facilitate the easy loading and unloading of containers below deck, the hull girders of these vessels have very low torsional rigidity. The hydrodynamic torsional loads may therefore cause considerable torsional deformation when the ships are sailing in oblique waves. The result is deformation of the hatch openings and large stress where the deck beams are attached to the side shell and especially large stress in front of the accommodation module. Existing ships often develop fatigue cracks in these areas. Linear hydrodynamic hull girder loads can be based on calculations in regular sinusoidal waves, but no consistent mathematical procedures can predict the torsional stress of flexible hulls in stochastic seas. Another serious wave-induced effect on the fatigue strength of flexible ship hulls is springing: a continuous excitation of the lowest natural frequencies of the hull girder due to the high-frequency components in the wave spectrum and due to nonlinear excitation effects. The natural frequencies of hull girders on ultra-large container ships are as low as 0.40 Hz (or 2.5 seconds in natural period), and springing can cause structural hull girder vibration in moderate seas with stress magnitudes that can cause significant fatigue damage to the hull within just a couple of years of operation. Again, no reliable procedure can predict these springing loads on large ships. To reduce the resistance and to increase the deck DTU

12 Fig Bow flare slamming and pressure distribution from computational fluid dynamics calculation area, the hull forms of large container ships are unique, with excessive bow flare and stern overhang. The result is that slamming loads become crucial for the integrity of the ship hull in harsh weather conditions. The calculation results shown in Fig. 1.3 are for slamming impact in a regular sinusoidal wave. No consistent procedure has yet been developed to predict the long-term distribution of slamming loads in irregular waves. Slamming loads cause large local impact forces on the hull, which may set up the plating and buckle internal frames. A second significant effect of slamming is transient vibration of the entire hull. This slam-induced hull girder vibration is called whipping (Fig. 1.4). The combination of the still-water loads and the wave-induced loads on the hull and the vibration response of the hull girder to bow or stern slamming impact can induce bending moments and shear forces that may cause the hull to collapse. One such example is the large container ship Napoli, which broke during a storm in the English Channel in 2006 due to whipping loads. This case has not been settled yet and poses a scientific challenge and a benchmark case for further developments. Fig. 1.5 summarizes some of these new challenges associated with the structural design of large flexible container ships such as the Emma Class series. Further research is strongly needed to design safe, large, fuel-efficient ships. Reliable probabilistic prediction of the maximum loads on large flexible ship structures during their lifetime operation in different seas is a major challenge that requires wave data, advanced nonlinear hydrodynamic codes, structural analysis models and advanced probabilistic tools. 150,000 Amidships vertical bending moment, significant wave height (Hs) = 3 m, peak wave periods (Tp) = 10.3 s, head sea Whipping 100,000 Vertical bending moment (kn m) 50, , , , Time (s) Fig Measured full-scale hull girder stresses showing slamming-induced hull girder whipping bending moments 10 TECHNICAL UNIVERSITY OF DENMARK

13 Stern slamming Fatigue stresses at hatch corners due to: Vertical bending Torsion Springing Whipping Hatch-opening distortions due to wave-induced loads Non-linear wave-induced stresses caused by: Vertical bending Torsion Springing Whipping Whipping acceleration due to slamming Fig Structural challenges in designing ultra-large container vessels Bow flare slamming The enabling analysis tools most needed include the following. Improving computational procedures for calculating fluid dynamics. These numerical calculation tools include free surfaces for predicting, for example, the flow field around the hull in calm water as well as in waves, calm water resistance and scale effects, viscous and scale effects on wavemaking, propeller inflow, propeller torque, thrust and cavitation phenomena, seakeeping ability, wave-induced pressure distributions in nonlinear confused sea, slamming pressures and maneuvering. Developing consistent theory on fluid structure interaction. Consistent procedures are needed for calculating coupling between impulsive sea loads and structure, the influence of hull flexibility on wave-induced loads, the structural effect of sloshing in tanks and wave-induced springing excitation of ship hulls. Procedures for nonlinear probabilistic design. An important characteristic of sea loads is their variability over time. Like the sea itself, the loads imposed by the sea are random in nature and can therefore only be expressed in probabilistic terms. Determining a single value for the maxi- DTU

14 mum loading to which a ship will be exposed during, for example, 25 years of operation in the north Atlantic is therefore not generally possible. The loads must be probabilistically represented instead. Further, because of the complexity of the structure itself and the limitations of analysis capability, the structural response of the structure cannot be absolutely accurately predicted even if the loading were known exactly. The design process must be based on reliability-based methods, and the degradation of the structures over time due to corrosion, fatigue cracks and other factors must be considered. Analytical tools within these research fields need to be developed further to ensure the safe design of completely new concepts such as ships for transporting CO 2 from power plants to the deep seafloor or offshore oil and gas fields and for the design of a possible future generation of even larger ships such as Malacca-max vessels carrying 18,000 TEU. As mentioned above, the most effective way to reduce CO 2 emissions is to increase the ships size and to reduce speed. However, additional measures can be applied for any given main dimensions and speed. Fig. 1.6 depicts several current research and development fields that explore opportunities for further reducing fuel consumption and emissions. The development of computational fluid dynamics in combination with tank testing has made progress in optimizing hull shapes to reduce calm water resistance. Other means of reducing the calm water drag resistance currently being analyzed include air lubrication of the ship bottom and new types of paint. Research on propellers has led to new optimized propeller shapes, devices to recover the rotational losses from the propeller stream and improvements in the water flow to the propeller. Computational fluid dynamics is also being used to explore alternatives to propeller propulsion such as fin propulsion and optimal strategies to use the wind as a possible auxiliary propulsion system in future ships either as rigid wings on the deck or as high-flying kites attached to the foremast. Even if the large two-stroke engines have very high thermal efficiency (about 50%), further fuel efficiency can still be gained. One possibility is to install a system to utilize the exhaust heat to generate power for a shaft generator. The integrated potential of technical solutions such as those mentioned above for improving energy efficiency and thereby reducing fuel consumption and CO 2 emissions is estimated to be 15-20% for existing ships. Kite-assisted propulsion Cargo and port operations Operations decision support Emissions Bow design Hull design Anti-fouling paint Drag reduction Air lubrication Main engine Auxiliary engine Shaft generator Aft hull design Propeller Rudder Fig Design elements for improving environmental performance with any given main dimensions 12 TECHNICAL UNIVERSITY OF DENMARK

15 Deep-water systems Fixed platform (0 500 m) Compliant tower ( m) SeaStar tension leg platform ( m) Floating production systems ( m) Tension leg platform ( m) Subsea system ( m) Spar platform ( m) Fig Concepts for deep-water oil and gas production platforms Offshore oil and gas production For the next many years, oil and gas will still be the main energy resources. Extracting the newly discovered oil and gas resources situated below the seabed at great water depths or in Arctic waters constitutes an enormous technological challenge. Offshore oil and gas production already comprises a significant portion of total production. Oil can currently be exploited at water depths up to about 3000 m (Fig. 1.7) using either tension leg platforms or spar platforms (the name spar comes from the mast analogy for sailing boats). Both concepts are huge floating units and require focus on hydroelasticity, fluid-structure interaction and probabilistic design procedures. The overall design goal of these platforms is to prevent direct resonance with dominant sea wave frequencies. Independent of the prevailing characteristic wave height, the wave energy is concentrated at wave periods between 5 and 25 s (Fig. 1.8). The tension leg platforms have natural vibration periods in the vertical direction of less than 5 s and exceeding 25 s in the horizontal plane; the spar platform has natural periods above 25 s for vertical and horizontal rigid body motions. The tension leg platforms have good motion characteristics from the tendons in tension, reaching down and connected to the sea floor. The caisson of the tension leg platforms must have enough buoyancy to yield the necessary tension in the tendons in extreme waves. The tendons are typically made of high tensile steel tubes about m in diameter with a wall thickness of mm. The design of the tendons constitutes a main challenge, as fatigue damage may occur due to high-frequency resonant vibration, conceptually similar to springing vibration in ships. Because the tension leg platforms move horizontally, the analysis has to include nonlinear or at least second-order forces. The spar platform is basically a long tubular column 25 to 50 m in diameter and m long. The bottom of the tubular column is at a depth where the pressure fluctuation due to the prevailing waves is very small. A more standard slack mooring system can therefore be used to keep the platform in place. The stability is ensured by having the center of gravity well below the center of buoyancy, using ballast in the bottom of the platform. As the diameter is fairly large, vortex-induced vibration can be a problem and helical strakes (Fig. 1.7) are usually needed especially in areas with strong currents. Estimating the vortexinduced vibration is a design challenge being solved by computational fluid mechanics codes. DTU

16 Resonance area Energy content Fixed structures 25 s Flexible structures Fixed structures 5 s 10 Flexible structures 8 Significant wave height (meters) 6 4 Fig Wave spectra for the north Atlantic Wave period (seconds) Properly estimating wave loads and wave load effects on these structures requires the inclusion of non-linear effects and, similar to ships, considering the randomness of the sea states. However, in contrast to ships, which are designed for worldwide trade, these offshore structures are designed for a specific location. Due to the high cost, wave buoys are typically used to collect detailed information on the wave environment at the intended location. A more accurate statistical description of the sea states can thereby be expected. The nonlinearities in the wave loads can often only be dealt with by time domain simulation in short-term sea states, which then has to be integrated into predicted long-term extreme values and estimated fatigue damage. A direct procedure can be too time-consuming even considering the huge cost of such structures. Several approximate although rather accurate procedures have been developed and validated. For short-term analysis, the first-order reliability methods developed within structural mechanics have proved to be very useful for predicting stochastic wave and wind loads. One outcome of the firstorder reliability methods is the most probable wave scenario leading to a certain response. This is illustrated in Fig. 1.9 for the extreme overturning moment for a jack-up drilling rig. This type of drilling rig is used for oil exploitation in water depths up to 120 m. Dynamic effects are important, as the fundamental period of the jack-up drilling rig at the maximum water depth and the zero-up crossing wave period are both about 9 s. The dynamic effects are responsible for the shape of the wave scenario showing the largest wave crest one period before overturning (at time = 60 s) and not, as in a quasi-static case, at the instant of over-turning. The critical wave scenario can be used in model testing to validate the calculation procedure, thus avoiding long model testing series with random waves. The feasibility of producing oil from offshore Arctic fields is being intensively assessed. For areas covered by multi-year ice, gravity-based concrete structures are believed to be the most feasible solution for water depths up to about m. For greater water depths, no feasible solutions exist today, as floating structures cannot be applied due to the ice. In areas with 1-year ice, steel structures as jack-up drilling rigs can be applied in areas with lighter ice. Subsea completion systems with wellheads situated on or below the seafloor are also being explored. The main research challenge related to Arctic off-shore exploration is related to estimating the ice loads and the movements of the icebergs and ice sheets. Ice scour on the sea floor is also a serious concern, as it determines the necessary coverage of pipelines and subsea completion systems. 14 TECHNICAL UNIVERSITY OF DENMARK

17 Wave elevation (meters) Aft legs Wave crest Sea level Forward leg Sea bed Wave direction H strictions imposed on the acceleration of the nacelle and on the yaw and pitch motions of the platform. Other offshore wind turbine concepts being evaluated are based on the tension leg platform. Concepts based on floating islands hosting several wind turbines are being developed. Finally, sustainable energy can be harvested from the waves. This field is still in infancy, and although many designs have been presented and evaluated in model scale, a real breakthrough awaits. One problem is that wave forces vary much more than wind forces. Designing wave energy devices such that they can cope with extreme wave loads and still be economically feasible for exploiting energy at the usual low wave heights thereby becomes a challenge. Another problem is related to the motion of such structures if they are floating. Generally, the motion will limit the effectiveness of systems based on waves running down a turbine. One way to circumvent this problem could be to combine floating wave energy devices with offshore wind turbines, such as the Poseidon plant (Fig. 1.11) Time (seconds) Fig Most probable wave scenarios leading to the overturning of a jack-up drilling rig at time = 60 s Sustainable offshore energy production Offshore wind turbines have multiplied rapidly in recent years. In shallow water, the standard monopile or jacket concepts from offshore oil production are usually applied. Scour and resonance frequencies are the main design problems but do not differ much from oil production platforms. For deeper waters, floating offshore wind turbines are being considered, although no full-scale construction is operating. The first to come will probably be the spar concept (Fig. 1.10). The height of the cylinder is about 165 m, of which 100 m is below the sea surface. It will hold a 2.3-MW wind turbine and can be positioned at water depths between 120 and 700 m. A new challenge compared with the platforms used in the oil industry is re- Fig A concrete spar-type floating offshore wind turbine DTU

18 Fig A proposed project for combining offshore wind turbine and wave energy plants However, such large structures have quite substantial wave loads and risk colliding with ships. Finally, marine growth on the water intake of such devices might pose serious maintenance problems. The future Climate change poses both risks and opportunities to all parts of the maritime sector. Since climate change requires urgent and extensive action by all parts of society to avoid the risk of serious damage to global prosperity and security, maritime research naturally strongly focuses on reducing emissions and on promoting new renewable energy sources. Shipping in general is the most energy-efficient mode of transport, and transferring land-based transport of goods to water-based transport has substantial climate benefits. However, due to increasing volumes transported, greenhouse-gas emissions from maritime activities are projected to comprise a significant percentage of total human emissions in the future without technological improvements. Oil and gas are still needed in the next 40 to 50 years, and an increasing proportion of the new di- Requirements for future marine structures E Activities transport of goods and people Resource exploitation oil, gas and minerals Defense and surveillance Sea wave, tidal and current energy harvest Using the sea Building maritime structures Maritime structures Vessels Platforms Systems Equipment Research activities Science and technology Recreation New lightweight strong materials New tools for predicting wave loads New structural models for large complicated structures Structural surveillance of structures during operation Reliability-based maintenace Fig Activities, requirements and scientific and technological disciplines that constitute the complex framework for the industrial exploitation of marine structures 16 TECHNICAL UNIVERSITY OF DENMARK

19 coveries will be from offshore drilling in much harsher environments than are customary today. Since the seas concentrate all the sustainable energy received from the sun, harvesting offshore wind energy and wind-driven wave energy will be emphasized more in the future. All this technological development requires maritime structures. The following apply to maritime structures (Fig. 1.12). The sea is a special environment that poses unique requirements through loads, response and materials. Prototype testing is normally not possible, and analysis and design must be based on first principles. Loads and response can only be expressed in probabilistic terms. This is a global activity, and international laws and regulations govern it. The investment required is huge. More to explore Mansour A, Liu D. Strength of ships and ocean structures. Jersey City, NJ, Society of Naval Architects and Marine Engineers, Jensen JJ. Load and global strength. Amsterdam, Elsevier, Moan T. Marine structures for the future a sea of opportunities. Marine Systems and Ocean Technology, 2005:1:1 19. Denmark has a maritime cluster, the Blue Denmark, and the infrastructure and engineering background to participate in the future technological development of maritime structures. This could be a future growth field with immense importance for the global environment. The authors Preben Terndrup Pedersen: MSc (1966) and Lic. Techn. (1969), Technical University of Denmark. Professor in Strength of Marine Structures, DTU Department of Mechanical Engineering. Sabbaticals: Harvard University and DNV, Norway. Head, DTU Department of Mechanical Engineering, Research: loads and response of ships and offshore structures, ship collisions and grounding, risk analysis. Jørgen Juncher Jensen: MSc (1972) and Lic. Techn. (1975), Technical University of Denmark. Professor in Marine Engineering, DTU Department of Mechanical Engineering. Head, Section of Coastal Maritime and Structural Engineering. Sabbaticals: regularly visiting researcher at the University of California at Berkeley. Research: hydroelastic and stochastic response of marine structures. DTU

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21 Sustainable energy catalysis is part of the solution Ib Chorkendorff DTU

22 Today most industrialized countries rely heavily on fossil fuel (more than 80%) for the energy for a modern and convenient lifestyle. A great challenge humanity faces is partly or fully shifting our energy supply from fossil fuels to sustainable energy sources. There are many reasons for this shift: lack of fossil resources, negative environmental effects, security of supply, independence and prices. Regardless of the reason, humanity living in balance with nature and trying to minimize negative environmental effects by promoting sustainability are desirable in the long term. Solving these issues and those connected to fossil fuels requires new and improved catalysts: materials that increase the efficiency of the current energy technology and increase the energy output from sustainable sources such as wind turbines, biomass and solar energy. Catalysts thus play a key role in securing both future energy supplies and the environment. Improving and designing new catalysts are the major objectives for the DTU Center for Individual Nanoparticle Functionality. Sustainable energy sources are limited and inefficient So why did humanity not shift to sustainable energy long ago? First, sustainable energy sources are inefficient with today s technology. Denmark is currently getting 13% of its energy from sustainable resources (10% from biomass and 3% from wind energy). Although considerable increases in capacity are planned, they cannot cover Denmark s entire energy demand. Wind energy is unstable because the wind fluctuates. Energy from biomass, the energy stored in plants by photosynthesis, is also a limited source because photosynthesis is inefficient. Even under the best conditions, no more than 0.5% of the solar energy supplied to the plants can be harvested. Many researchers and engineers, both nationally and internationally, are therefore searching for other, more efficient and stable sustainable energy supplies. In this quest, they have found one shining source. The Sun: an unlimited but expensive energy source The Sun is an everlasting (at least on our time scale) and plentiful energy source. In less than 2 hours, the Earth receives enough solar energy to cover global human energy consumption for 1 year. This energy can be harvested as electricity by photovoltaic (solar) cells and used locally or transmitted to consumers farther away. Experiments with solar cells have demonstrated much higher efficiency per land area than can be obtained by growing biomass. If a realistic 10% efficiency is assumed, this only requires 5% of Denmark s land area and not necessarily farmland, enabling such a solution without influencing food production. Solar cells, however, have several problems. The amount of sunlight arriving at the Earth s surface is not constant but depends on location, time of day and year and weather conditions. Further, solar cells are very expensive and thus currently not viable economically, although improved technology and rising energy prices may alleviate such challenges. The relationship between the price of fossil fuels and the viability of new sustainable but also more expensive energy technologies applies to all new energy approaches. Both fossil fuels and more sustainable alternatives pose several limitations and concerns for which there are no perfect solutions. Fossil fuels are limited and harm the environment; sustainable energy sources are inefficient, expensive and unstable. Thus, the present use of fossil fuels needs to be improved, surplus sustainable energy needs to be stored and new renewable energy technologies need to be developed. This work of improvement and invention highly depends on the ability of catalytic materials to accelerate and lower the energy consumption of the chemical reactions involved in energy conversion and storage, and this requires new and improved catalysts. The following sections focus on the use of and research in catalytic materials for existing and future energy supplies (Box 2.1). Fossil fuel reserves are still extensive Fossil fuels will be the primary energy resource for decades to come since no economically competitive or sufficiently efficient sustainable energy technology can take over. The reported estimates of reserves amount to about 40 years of oil, 60 years of natural gas and hundreds of years of coal. These numbers have been fairly constant during the past 20 TECHNICAL UNIVERSITY OF DENMARK

23 Box 2.1. What is catalysis? A catalyst is a material that speeds up a chemical reaction without being consumed in the reaction itself. Most people benefit daily (and perhaps unknowingly) from the use of catalytic converters in cars that remove harmful emissions of CO and NO by facilitating chemical conversion into CO 2 and N 2 (Fig. 2.1). This makes the reaction pathway from reactants to products much easier, and the reaction runs faster and uses less energy. Since chemical reactions only take place on the surface of materials, one way to improve a catalyst s efficiency is Potential energy A Reactants + B With catalyst Without catalyst to increase its total surface area. The smaller the particles, the larger the fraction of the material is on the surface. Thus, materials in the shape of nanoparticles (on the order of 10 9 meters in diameter) have a much larger total surface area than the same amount of material in the shape of microparticles (on the order of 10 6 meters in diameter) and are thus much more efficient than microparticle catalysts for the same amount of metal. Catalysts have important roles in improving existing energy technologies and in creating new ones. Catalysis is involved in 20 30% of total economic production and plays an essential part in many industrial processes including pharmaceuticals, food, catalytic conversion and 90% of all chemical production. Unfortunately, a predominant problem is that most important catalysts comprise expensive and scarce materials such as platinum and palladium. Thus, improving catalytic processes requires finding less expensive and more abundant materials with the same or higher catalytic activity than current catalysts. Surface sites A B AB Reaction pathway Products AB Fig Catalysts typically consist of active sites on metal or oxide particles. The catalyst reduces the energy needed for the chemical reaction by adsorbing the reacting gases (reactants) onto the active surface sites on the particle, thus bringing the gas molecules into closer contact. decades, indicating that new resources are still being found. The argument of peak oil production is often raised, but the primary product produced from oil gasoline for transport may be produced from natural gas or even coal. As oil prices go up, catalytically converting first natural gas and later coal into liquid fuel will become profitable, extending gasoline resources well beyond 40 years. Thus, fossil resources are available, although the price will increase as the reserves shrink and they become increasingly difficult to extract. Nanocatalysts improve existing technologies Catalysis is the key technology for efficiently converting fossil fuels into useful chemicals or converting natural gas and coal into gasoline. As this needs to happen for many years to come, there are good reasons to improve and optimize these processes as much as possible. The processes require catalytic materials that have a huge surface area per mass to maximize the active area of metals that are often scarce and expensive. Nanoparticles are therefore the preferred size for catalytic materials. DTU

24 The shape and size of the nanoparticles are often essential for their functionality, and large particles may not work at all. Fig The experimental hall of the DTU Center for Individual Nanoparticle Functionality funded by the Danish National Research Foundation. The experimental facilities rely heavily on a surface science approach for studying the active surface of catalytic materials at the atomic level under well-controlled conditions while linking to real operating conditions. The hall hosts eight instruments specially designed for various investigation, each representing an investment of 400,000 to 1 million. The instruments are used for basic research, often in close collaboration with companies manufacturing or using catalysts such as Haldor Topsøe A/S and IRD A/S. Optimizing catalytic reactions saves energy Understanding what optimizes the nanoparticle reactivity for a specific reaction as a function of size and shape is a scientific challenge but also the key to optimizing many catalytic reactions and to designing new processes. The importance of this understanding is illustrated by recent work in which the Center for Individual Nanoparticle Functionality (Fig. 2.2) and Center for Atomic-scale Materials Design of the DTU Department of Physics closely collaborated with Haldor Topsøe A/S in studying two essential steps in hydrogen production both experimentally and theoretically in great detail. Producing hydrogen is key in producing both liquid fuels such as gasoline and alcohols and ammonia (fertilizer). The world uses about 100 million tons of nitrogenous fertilizer per year, consuming at least 1% of total world energy production. Thus, improving the process of producing hydrogen is very significant. Hydrogen is manufactured by converting methane (CH 4 ) and water into a mixture of CO and hydrogen by the steam-reforming process. This is also the first step in producing liquid fuels from natural gas and the route for producing large amounts of ultra-pure hydrogen for synthesizing ammonia. In the latter case, CO can be further converted by water into CO 2 and hydrogen through a low-temperature process called the water-gas shift reaction. CO 2 is subsequently removed, and hydrogen is the product. However, even parts per million (ppm) levels of residual quantities of oxo-compounds such as CO and CO 2 in the synthesis gas will harm the ammonia synthesis catalyst substantially and must therefore be removed by the inverse steam-reforming process. This converts both gases back to methane CH 4 + H 2 O CO + 3H 2 Steam-reforming process CO + H 2 O CO 2 + H 2 Water-gas shift reaction CO + 3H 2 CH 4 + H 2 O Methanation CO 2 + 4H 2 CH 4 + 2H 2 O } 22 TECHNICAL UNIVERSITY OF DENMARK

25 and water, and then the oxygen can easily be removed from the gas. This process is called the methanation reaction and is represented by the last two reactions. Only a small proportion of the hydrogen is sacrificed in this manner to produce ultraclean hydrogen. Shape and size matter Catalysts are involved in all these steps. By lowering the activation energy, the catalyst makes the chemical reactions run faster and use less energy. We examined the methanation reaction and found that the first rate-limiting step of the reaction is the dissociation of CO into C and O atoms bound to the active site on the catalyst surface. Speeding up this step requires understanding exactly what influences the reactivity of the active site. Surprisingly, the reactivity turned out to depend on the geometry of the active site on the catalytic nanoparticle. Instead of nanoparticles, single crystals are often used as model systems when studying catalytic reactions. A single crystal has all atoms positioned in identical surroundings in a crystal lattice. It is therefore a better model system than the more disordered nanoparticles for investigating the influence of the local atomic arrangement on the catalytic activity. Comparing measurements of CO dissociation on flat and stepped nickel (Ni) singlecrystal surfaces accordingly (Fig. 2.3) clearly showed that the monoatomic steps play an essential role in the reaction. When the stepped crystal surface is exposed to a fixed dose of CO, a certain amount will dissociate, leading to carbon on the surface at a rate that can be monitored. However, when the step sites are blocked with sulfur, carbon formation is reduced, clearly demonstrating the importance of the step sites. Theoretical calculations backed the experimental data, showing the exact same picture: CO cannot dissociate on the planar crystal terrace but need ensembles like those created along a monoatomic step to proceed. This observation should naturally also affect the real catalyst, and this was confirmed by investigations of the influence of the particle size and the reactivity on real catalysts measured at Haldor Topsøe A/S. These investigations of how the structure of the catalytically active site influences a step in the methanation reaction mainly illustrate the importance of identifying the active site on a catalytic material to determine exactly what to modify to improve the catalyst. This also illustrates another important point: fabricating catalytic nanoparticles requires not only making small particles but also particles with the right shape and size to optimize the catalytic activity of the sites essential for the reaction. This insight is very important for manufacturing new and more efficient catalysts, not only for ammonia production but for all catalytic reactions including more efficient conversion of fossil fuels. Power after 4:00? Learn how to store Most sustainable energy is in the form of electricity and fluctuates depending on sun, wind and other Carbon coverage (monolayers) Clean surface With 0.05 monolayers of sulfur Steps Terrace Dose time (minutes) Fig The dissociation probability of CO on a flat and stepped surface of an Ni single crystal is measured as a function of dose time. The crystal is cut so the surface exposes a monoatomic step for every 26 rows of atoms (insert, upper right). The CO dissociation rate is measured as the amount of C deposited on the surface. The blue curve shows CO dissociation on unblocked steps and the resulting C deposition. When the steps are blocked by sulfur (green flat curve), no C and thus no dissociation is seen at all: the terrace atoms are not active. This observation clearly shows that steps play a crucial role in dissociating CO. DTU

26 weather conditions. If sustainable energy is to constitute most energy in the future, ways to average out this source are strongly needed to secure a stable energy supply 24 hours a day. When there is a surplus of electricity due to favorable production conditions such as windy weather or low energy consumption at night, the energy needs to be stored. When electricity generation decreases or consumption increases, energy can be released from storage. The simplest way of storing electrical energy is in batteries. Unfortunately, batteries are limited by capacity and charging time for such huge amounts of energy. Other storage methods than batteries must therefore be considered, preferably transforming electrical energy into energy stored in chemical bonds in liquid fuels such as alcohol or hydrogen. In theory, converting electrical energy into chemical by splitting water into oxygen and hydrogen is very simple. This is called electrolysis. The hydrogen produced can be stored and converted back to electricity when needed or, like natural gas, be transported out to the consumers and there oxidized back to water in a fuel cell, thereby generating electricity. This sounds very simple and tempting, especially if the two processes could take place in the same cell: a combined electrolysis (H 2 O to H 2 and O 2 ) and fuel cell (H 2 and O 2 to H 2 O). In this way, hydrogen could be produced, stored and used again right on the spot. From water to water it does not get any better. Inadequate fuel-cell efficiency Unfortunately, such processes lose energy, and only about 30% of the energy is available to consumers. Understanding this requires examining how a proton-exchange membrane fuel cell operates (Fig. 2.4). The major energy losses in this process appear because the hydrogen and oxygen on the anode and the cathode (respectively) are not ideally adsorbed and desorbed. Thus, the acid fuel cell only has an efficiency of 45%. Electrolysis is the reverse process, and alkaline electrolysis has an efficiency of about 70%. Combining the two types of cells leads to an overall efficiency of 30%, as mentioned above. Unfortunately, the two types are not compatible, and a cell therefore needs to be developed that can operate reversibly, reducing capital investment. This requires new efficient catalysts for the electrolysis. Less expensive catalysts are needed Fortunately, fuel cell efficiency can be improved (Fig. 2.5). In contrast to power plants, which have a maximal theoretical operating efficiency for electricity of about 45% at reasonable operating temperatures, the theoretical efficiency for hydrogen oxidation is about 90% at 80 C, which is the maximum operating temperature for a proton-exchange membrane fuel cell. The fuel cells are currently not much more efficient than the power plants, as indicated by the blue band in Fig. 2.5, but they are not limited by thermodynamics in the same manner. 1 Hydrogen fuel is fed to the anode on one side of the fuel cell while oxygen is channeled to the cathode on the other side. Pt clusters 2H 2 4e + 4e Electrolyte 4e Pt clusters Air 3 The proton-exchange membrane allows only the protons to pass through it to the cathode. The electrons must travel along an external circuit to the cathode, creating an electrical current. 4H + 4H + 4H + 2 At the anode, a platinum catalyst causes the hydrogen to split into positive protons and negatively charged electrons. Anode O 2 Cathode 2H 2 O Heat + water 4 At the cathode, protons and electron combine with oxygen to form water. Fig Main operating mechanism of a proton-exchange membrane fuel cell 24 TECHNICAL UNIVERSITY OF DENMARK